As a technique to improve an output of an engine, a method (supercharging) of compressing intake air with a supercharger and supplying an engine with the compressed intake air is known, and widely used in various engines. When the operational state of an engine changes suddenly, for instance, a supercharger may transiently enter an abnormal operation state called surging. When a supercharger enters a surging state, it is no longer possible to compress intake air. Further, if a supercharger enters a surging state often, it may lead to breakage of the devices. Thus, a supercharger needs to be controlled appropriately so as to avoid entering a surging state as much as possible.
Typically, as a technique to control a supercharger, a feedback control is performed on a supercharger so that an operational point on a compressor map does not enter a surge region, on the basis of information obtained by sensing the intake flow rate and the rotation speed, for instance. In other words, a supercharger is feedback-controlled on the basis of the current position of an operational point on a compressor map. For instance, Patent Document 1 discloses performing a feedback control on the turbo rotation speed so that the actual turbo rotation speed detected by a turbo rotation speed sensor matches a target turbo rotation speed corresponding to the current operation state, thereby preventing the operational point of a turbocharger on the compressor map from entering the surge region.